Fuel cells are simple devices capable of continuously converting stored chemical energy into electricity. In general, a fuel cell comprises a pair of electrodes separated by a semi-permeable electrolyte membrane. At one of the electrodes, the anode, oxidation of input fuel occurs. The fuels used depend on the type of fuel cell system as described below and include, but are not limited to, materials such as glucose, methanol, ethanol, hydrogen, formic acid, carbon monoxide, and simple hydrocarbons like methane, propane, or butane. The electrons extracted from the fuel are transferred as electric current through an external circuit to the second electrode, the cathode, where an input oxidant is electrochemically reduced. Typically, oxygen is the oxidant and it is reduced in a four-electron process to water. The semi-permeable membrane functions to separate the fuel and oxidant. In addition, transfer of protons or other ions through the semi-permeable membrane ensures charge balance and completes the circuit.
Although a variety of different systems have been developed, fuel cells can be generally classified as one of three types based on the electrolyte membrane type and power output operating conditions. Biological fuel cells employ electrodes modified with enzymes or microbes that function as the electrocatalysts. Although these cells usually employ simple ion exchange membranes, cells that require no membrane can also be fabricated. In these cases, cross-reactions of the anolyte fuel and catholyte oxidant with the opposite electrodes are prevented due to the high specificity of the bioelectrocatalytic reactions at the electrodes. Net chemical reactions for these cells are usually quite simple, for example, eventual oxidation of glucose to carbon dioxide with the corresponding reduction of oxygen to water, and power outputs are small, typically ranging from microwatts to milliwatts.
Fuel cells at the other extremum of power output generally utilize ceramic-based or solid electrolyte membranes, operate at temperatures greater than ˜600° C., and can reach power output levels of kilowatts to megawatts with fuel power conversion efficiencies exceeding 40%. For example, solid oxide fuel cells operating at temperatures exceeding ˜800° C. utilize a solid oxide electrolyte material that transports oxide anions through the membrane to react directly with fuel at efficiencies approaching 60%. Likewise, molten carbonate fuel cells utilize a porous ceramic membrane containing a mixture of molten carbonates as an electrolyte and can reach similar operating efficiencies. Both types of cells can utilize a variety of fuels and, with proper operating configurations, achieve power outputs approaching ˜100 megawatts.
Polymer electrolyte membrane (PEM) fuel cells, also known as proton exchange membrane fuel cells, represent the third general class of fuel cell systems. PEM fuel cells use a solid polymer as the electrolyte membrane in combination with porous carbon electrodes containing Pt catalyst. Nafion® and related ionomers having good proton conductivities are usually used as the polymer electrolyte membranes. These systems typically generate power at levels ranging from watts to kilowatts and operate at temperatures ranging from ˜70° C. to ˜200° C. Their light weight, durability, and respectable power densities compared to other types of fuel cells make them attractive candidates for both portable (e.g., automotive) and stationery (e.g., home power) applications. A majority of the research in this area involves fuel cells using either methanol or hydrogen as fuels. PEM fuel cells using methanol as fuel and generating power via a methanol oxidation reaction (MOR) will hereafter be referred to as direct methanol fuel cells (DMFCs), while those fueled by hydrogen and generating power via the oxidation of hydrogen will be labeled as hydrogen fuel cells (HFCs).
As a liquid, methanol is a more attractive fuel than hydrogen, at least for automotive applications, since it is more readily handled and transported using the existing petroleum hydrocarbon infrastructure. Unfortunately, oxidation of methanol in a PEM fuel cell invariably produces carbon monoxide as an intermediate oxidation product, which can poison the Pt catalyst and significantly reduce the power output of the cell. In contrast, hydrogen presents obvious dangers regarding storage and handling but is oxidized cleanly in a fuel cell to protons, which are ultimately captured as water.
PEM fuel cell electrodes are heterogeneous supported catalyst structures whose electrocatalytic activities are greatly affected by the microenvironment surrounding the catalyst particles. Electrodes are usually fabricated by intimately mixing a colloidal Pt electrocatalyst, together with a small amount of Nafion® or other ionomer, in an electrically conductive porous Vulcan carbon matrix. The resulting mixture is usually applied as thin layers to both sides of a solid Nafion® film to prepare the separate anode and cathode electrodes, which are fixed by hot-pressing or related techniques to complete the membrane electrode assembly (MEA).
The power output available from a particular PEM fuel cell type and its power conversion efficiency are functions of the structure and composition of the MEA. Consider, for example, a HFC system. During operation of the cell, electrocatalysis is thought to occur most efficiently at a triple phase boundary (R. O'Hayre, D. M. Barnett, F. B. Prinz Electrochem. Soc., 152, A439 (2005)), where H2 fuel contacts the junction formed by a colloidal metal catalyst particle with the ionomer and carbon support. Specifically, the hydrogen oxidation reaction (HOR) occurring at the anode (eq. (1)) is thought to be facilitated at the triple phase boundary by efficient removal of the electron and proton products from the catalytic particle sites by the carbon support and ionomer, respectively, minimizing the possibility of a reverse reaction. Likewise, enhanced transport of electrons and protons via the conductive carbon and ionomer species, respectively, to catalytic particle sites at triple phase boundaries has been proposed to facilitate the oxygen reduction reaction (ORR) occurring at the cathode (eq. (2)). Similar models have been proposed for operation of DMFCs. Consequently, optimization of the MEA structure via changes in fabrication materials and techniques to maximize the occurrence of such triple phase boundaries represents a continuing focus for research to improve fuel cell performance.Anode: H2→2H++2e−  (1)Cathode: O2+4H++4e→2H2O  (2)
Remediation of inefficiencies specifically associated with the properties and performances of the polymer electrolyte membrane and the Pt nanoparticle catalysts to improve fuel cell performance comprise two additional important research areas. For example, while Nafion® polymer electrolyte membranes efficiently transport protons required for successful cell operation, the internal electrical resistance of these semi-permeable membranes and fuel crossover through them can reduce cell performance. The electrochemical performance of Pt catalysts is also limited, especially at the cathode where the ORR suffers from slow kinetics requiring high overpotentials and Pt loadings too high for viable commercial use. Improvements in polymer electrolyte membranes to date have focused primarily on the use of new materials and modification of membrane structures to address PEM resistance and fuel crossover issues. Current strategies for improving the electrocatalytic activity of Pt-based catalysts mainly consist of combining Pt with other transition metals, replacing Pt altogether with other less expensive metals, or tailoring the Pt particle size to control the relative fraction of Pt surface atoms.
While these efforts have yielded considerable improvements in PEM fuel cell performance with regard to power density, efficiency, durability, and stability, further efforts and new research paradigms are still required to realize commercial systems capable of competing economically with current power sources. In this disclosure, we present a new paradigm for the development of PEM fuel cells having superior performance characteristics based on the fabrication of nanostructured electrode architectures using well-defined Pt nanoparticle (NP) catalysts whose electrocatalytic activities are determined via strict control of particle morphology and surface functionalization.